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This industrious little craft is the most recent in a long line of working ships and can be seen in various guises throughout the Solar System and beyond. Wherever there is industry, the Mule Multirole tug is to be found busily carrying out a wide range of functions. The basic design has scarcely changed in the last forty or fifty years as there seems to be little to be done to improve it. The modifications that distinguish the various models have mostly been concerned with improvements to its ancilliary equipment or power system. Probably the most familiar version is the one which can be seen in all the older orbital spaceports and transfer stations, where tugs are employed to assist the docking of liners and freighters. The newer stations are of course equipped with automatic docking facilities but even these usually have their contingent of Mules for maintenance, loading and emergency work. These models are easily identified by the large external pod beneath the main hull, housing the powerful directional electromagnet used to manoeuvre the docking vessel.

The best places to see the Mule's many variants are undoubtedly the fabrication yards, particularly those orbiting Mars. Here they can be observed executing most of the functions for which they can be equipped and our illustration shows two gantry tugs manhandling a giant spar section for a new orbital manufacturing complex.

The lower half of the central pylon is fixed and contains the bridge and control centre of the ship, the gantry turret forming the upper part. On those models equipped with photowelding and cutting lasers the layout is almost identical, but without the articulated jib. The laser projectors are mounted on slender stalks projecting from the rotating band, here carrying the main hydraulic pistons. Cableor pipelaying tugs are again almost identical but are easily distinguished by the large drum mounted above the bridge, giving the ship a markedly topheavy appearance. The two plates projecting from beneath the ship are electromagnet grips and are used for either carrying less bulky loads or for anchoring the vessel to a surface.

Other versions are sometimes difficult to identify as modifications are not readily apparent, such as fuellers or those designed for handling fissionable materials where heat or radiation resistant hulls are used.